-Fix issue 4416

See [issue 4987](https://github.com/diaspora/diaspora/issues/4987) and [4416](https://github.com/diaspora/diaspora/issues/4416)
Redcarpet 3.2.0 has the same problem. See [render](https://github.com/vmg/redcarpet/blob/340efa531e669b5e532781e906789ae5cf98562d/test/stripdown_render_test.rb#L35) function is not designed to remove the brackets surronding the link.

The code of Redcarpet must be directly changed or my PR must be used but I don't see how to do better...